Applicants"" invention relates generally to the field of programmable controllers and more particularly to a system for the exchange of timexe2x80x94critical information between control devices coupled to an intranetwork such as would be common in the fields of factory automation and industrial process control.
Real-time control requires a high degree of determinism over a general purpose network. Determinism is a measure of ensuring that a real-time event will be handled in a known period of time. Data flow load control approaches based on voluntary bandwidth allocation have been tried to increase the level of determinism. Typically a network is set up to budget a particular amount of transmission time per station per unit time, and as long as all stations abide by the restriction, predictability is achieved. Two factors impeded this goal, configuration complexity and the fact that secondary or occasional network participants are not bound by any agreements. Standard networks such as MAP have a long set of parameters which have to be allocated and agreed among the stations for interoperability to be achieved. In most cases, interoperability fails because of a mismatch of the parameters. A simple file transfer or database lookup by someone""s portable computer could inadvertently disrupt the fragile assumptions about transmission bandwidth. Networks such as MAP handle this situation by not allowing laptop computers and other certain devices to connect to it to prevent any such problems. The use of communications techniques in automation products is typically stratified into at least 3 layers.
At the highest level are conventional data processing communication networks, designed to exchange information such as data files, electronic mail, and reports, and more recently to support widespread dissemination of information using the Internet and World Wide Web. Typical exchange of information is not repetitive, but it is transferred on demand and the level of loading of the network is unpredictable with varying delivery times. Examples of such networks are Ethernet, IBM Token Ring, Fiber Distributed Data Interface, the X.25 international packet switch network and many offerings from telephone companies such as Asynchronous Transfer Mode. General purpose network protocols using this hardware include the increasingly dominant TCP/IP, and Novell IPX, Digital Equipment""s DECNET and others. The TCP/IP-Ethernet combination, in particular, is the most widely deployed computer network interface in use, and therefore has minimum cost to implement and support.
At the lowest level are specialized data moving buses, designed to allow a control device such as a computer or a Programmable Logic Controller to exchange information with its sensors and actuators. These buses are designed to carry the same information repetitively, and can therefore guarantee a maximum time between the value of the information changing, and recognition of the changed values by the partner devices. Examples of such technologies are remote I/O networks, Small Computer System Interface (SCSI), and various backplane bus extender techniques from many computer and automation vendors. Typically the messages carried are highly specialized and do not get copied directly onto the general purpose networks.
In a middle layer are a number of fieldbus solutions which accommodate supervision and updating of control devices. There are many approaches which compete with each other, and offer limited compatibility with each other. Examples include Siemens PROFIBUS, Schneider Automation""s FIP and MODBUS PLUS., Allen Bradley""s DEVICE NET, and Echelon Corp""s LONWORKS. All of these network approaches require dedicated wiring and troubleshooting techniques but allow some mixing of control data exchange and equipment interrogation.
It would be desirable to develop an automation control system whereby these problems are minimized, using the same type of general purpose networks (Ethernet, Token Ring, ATM) at all three levels, allowing automation devices to use commercial network techniques, but still retain the security and performance characteristics of specialized industrial networks.
Accordingly, the principal object of the present invention is to provide an interface between an industrial control system and a general purpose network such as Ethernet.
Another object of the present invention is to provide an interface between the general purpose network and the industrial control system that will allow the transfer of real time control data with guaranteed delivery times.
A further objective of the invention is to provide an interface between the general purpose network and the industrial control system that will carry on-demand traffic from computer systems, operator terminals, and alarm systems.
In the preferred embodiment of the invention, the invention allows for control of a Programmable Logic Controller""s (PLC) functions over a non-deterministic network connection. Although the network is not necessarily real time, a proxy server, which normally is used to provide a network firewall to solve security problems, provides a high degree of determinism. It does this by taking the role of a TCP/IP router and by a combination of routing, firewalling, and limiting the network throughput. It is configured as a communication interface between a real time and an non-real time portion of the complete system: It is installed outside of the network bridge and serves to restrict unnecessary traffic between the real and unreal time elements. It does this by controlling the rate at which messages are forwarded from the non-real time to the real time portion of the network, keeping the loading of the real time portion stable regardless of the external non-real-time communication demand. This allows the non-reel time network which is normally non-deterministic, to attain determinism by maintaining a desired level of probability for a successful message transmission in a fixed time period.
Real time data is preconfigured and exchanged in a regular sequence, such that for any item of information which is being repetitively updated, the cyclic update period is known. The length of any communication message necessary to transmit this information, plus any sequencing and acknowledgment overhead, is calculated in advance and aggregated. Each device with authority to transmit on the shared medium is given a budget consisting of a maximum transmission quantity in unit time, calculated so that the total transmission quantity in unit time is limited to some fraction of the maximum transmission capability of the network. In addition, the maximum length of an individual transmission is defined. All communication from devices whose traffic loadings cannot be so controlled is arranged to pass through a proxy device in order to gain access to the deterministic network, and that proxy enforces the budget limits by introducing deliberate delays to the request messages if necessary. The appropriate budget limits as percentages are dependent on the chosen network topology. A simple Ethernet can then be made equivalent to a dedicated fieldbus by exercising control over the loading of the network.
Other features and advantages of the invention, which are believed to be novel and non-obvious, will be apparent from the following specification taken in conjunction with the accompanying drawings in which there is shown a preferred embodiment of the invention. Reference is made to the claims for interpreting the full scope of the invention which is not necessarily represented by such embodiment.